The present invention relates to a process for the production of bi-metallic contact rivets, with an extra-thin precious metal layer, through cold pressure welding operation. Such a process is known in the pertaining art (for example, from CH-PS No. 386 212, both the execution examples according to FIGS. 8-14 and according to FIGS. 15-21.) In the execution of the known process, two unequally long and differently composed wire segments with congruent cross sections are separated, arranged behind one another in a guide bush, and cold pressure welded against one another through pressure and the enlargement of striking surfaces, on the cutting surfaces which lie against one another. Thereby, the enlargement of striking surfaces will be used at the same time for the formation of the rivet head, so that the abutment--against which the wire segments are pushed out from the guide bush by an ejector pin--already occupies the contour of the rivet head, which serves at the same time as the header, so that the rivet head is formed by the upsetting of the wire segments between the ejector pin and the abutment at the same time, so that only a half-completed slug is fashioned first of all by cold press operation; the head of this slug receives its final form, in a second deformation step, through a special header. Usually, the longer wire segment consists of copper and the shorter one of silver. The copper is inserted for the formation of the rivet shaft as well as for the formation of part of the rivet head which is further back; the costly silver, on the other hand, is only inserted for the formation of the contact layer proper.
In comparison with contact rivets of solid silver, bi-metallic rivets with a copper shaft and with contact surfaces of silver bring about a considerable saving of silver. The on-going cost increases of precious metals has brought it about that one is now most concerned to further reduce the precious metal insertion in bi-metallic rivets. So, for example, it has been proposed to insert the precious metal in the center of the contact surfaces only; however, such kinds of bi-metalllic contact rivets are, on the one hand, relatively expensive to produce, and on the other hand they only bring about an apparent savings of precious metal, since a contact surface requires a certain minimum size for any given purpose of insertion. A substitute of precious metal in the border area by base metal would unacceptably affect the switch behaviour.
One is thus concerned to keep the precious metal layers on the bi-metallic contact rivets as thin as possible. As long as one starts with plated, cord-like material in the production of bi-metallic contact rivets, then it is no problem to keep the layer of precious metal agreeably thin. However, the production of bi-metallic contact rivets from cord-like material is so expensive, that such rivets are much more expensive than rivets which are produced through the cold pressure welding from wire. In the last mentioned process, however, the precious metal layer can not be made suitably thin. The reason lies in this, that it is possible to cut off and handle suitably short wire segments. According to experience, one needs in a silver wire with the diameter D a minimum length of the wire segments of approximately 0.5D to 0.8D, where the lower value 0.5D is valid for very thick wires, and the higher value 0.8 D for very thin.
At the base of the present invention lies the task of making available a process suited for mass production which permits bi-metallic contact rivets, with thinner precious metal layers than previously, to be produced by means of cold pressure welding operations.